Antennas for VHF/UHF hand-held portable equipment, such as pagers, portable telephones and transceivers, must naturally be small in size, light in weight, and compact in structure. Nevertheless, there is a growing tendency for portable equipment to be made continuously smaller as the demand for mobile communication rapidly increases. Consequently, there has become an increasing demand for antennas that are suitable for increasingly smaller portable equipment.
It is well known in classical antenna theory that antenna dimensions are related to the wavelength of the antenna's resonant frequency. It is also well known that the antenna's electrical performance degrades as its dimensions become physically small when compared to its wavelength. Therefore, requirements on antenna performance are becoming increasingly severe in order to place resonant antennas in smaller and smaller packages while avoiding degraded performance as the antenna size becomes smaller. Because of size restrictions, conventional pager antennas use a tuning circuit in conjunction with a non-resonant antenna element that is very small with respect to the wavelength of the pager receiver frequency.
FIGS. 1a-1c illustrate a conventional antenna 5 used in many modern pager systems. Antenna 5 includes a ground plane 10, a dielectric substrate 12, and a non-resonant loop 14. Antenna 5 is configured such that ground plane 10 and non-resonant loop 14 are disposed perpendicular to each other with dielectric substrate 12 separating them. Ground plane 10 is a zero voltage potential conductor for antenna 5 which serves as an electrically conducting plane. Non-resonant loop 14 functions as a conductive element for antenna 5 that receives electromagnetic waves. Ground plane 10 and non-resonant loop 14 are typically connected to the ground potential (zero volts) and the desired incoming signal line, respectively, of the receiver circuitry (not shown).
Most pager systems are oriented to receive signals from a base station transmitting signals of vertical polarization. Polarization refers to the orientation in space of the electric field vector of an electromagnetic wave as received or radiated from an antenna. If both an antenna and base station have the same polarization, a match occurs (i.e., signals are received). Cross-polarization occurs, however, if an antenna and base station do not have the same polarization (e.g., base station has vertical polarization and antenna has horizontal polarization). If cross-polarization occurs, most of the transmitter antenna's signal will be undetected by the receive antenna.
All parts of antenna 5 must be made as small as possible in order to fit into a pager. Because of such size restrictions, non-resonant loop 14 may be very small with respect to the wavelength of the receiver circuitry tuned frequency. For example, antenna 5 may be required to be as small as one twentieth of a wavelength for some applications. However, classical antenna theory states that antennas work best when their dimensions are an appreciable portion of a wavelength; most antennas require a size ranging from one fourth to one half of the wavelength at the frequency of interest. Thus, an antenna as small as antenna 5 is an inefficient radiator that suffers poor input impedance and low radiation resistance.
Considering that antenna 5 is an inefficient non-resonant radiator, matching circuit components must be added in order for antenna 5 to receive the desired frequency. Coupling a matching circuit to antenna 5 exceedingly suppresses the frequency bandwidth (i.e., the frequency range over which operation is satisfactory). Matching circuit components limit the bandwidth as they tune antenna 5 to one frequency only. As a result of requiring a matching circuit, conventional pager systems have very low bandwidth frequencies.
The addition of a matching circuit also limits the gain of conventional antennas such as antenna 5. The gain is limited in antenna 5 by the absorption of some of the received energy by the matching circuit before it can be passed on to the receiver circuitry. Furthermore, the small physical size of antenna 5 yields a small energy capture area for electromagnetic waves. Due to the minimal gain of antenna 5, a pager must remain within a close distance of a base station in order to receive signals. Consequently, an abundant number of base stations are required to efficiently operate a network of pagers.
Further, in conventional antennas, the operating frequency may be changed by small changes in the physical materials of the antenna and by small changes in the matching circuit components. Thus, if material tolerances of antenna 5 and the matching circuit vary enough from one production lot of pagers to another, each antenna 5 may need to be individually tuned to ensure that the pager will receive the desired frequency. Having to retune each pager causes an increase in both manufacturing time and expense.
FIG. 2 is a plot of the frequency-gain-bandpass amplitude response for antenna 5. The plot represents the gains and frequencies at which antenna 5 operates. Gain represents the ratio of power density radiated by an antenna in a specific direction as compared to an isotropic antenna which radiates energy in all directions equally. Thus, at a given frequency, the higher the amplitude response the easier an electromagnetic signal is received by antenna 5 at that frequency. The number 1 on the plot represents a designed frequency (i.e., frequency of interest) of 940 MHz. The gain at this frequency is -4 dBi (4 dB below the gain of an isotropic radiator) at which antenna 5 is tuned. The numbers 2 and 3 represent the lowest and highest frequencies, respectively, at which antenna 5 has an amplitude response that is 3 dB below the design frequency. Antenna 5 will not efficiently receive electromagnetic waves at frequencies outside of this region. As the frequency is displaced from the design frequency, the performance of antenna 5 is greatly reduced. For example, antenna 5 has an frequency bandpass amplitude response spanning 20 MHz before the response falls below 3 dB of the maximum amplitude response (930 MHz and 950 MHz at -7 dBi). Accordingly, antenna 5 will not operate if the frequency is negligibly varied and outside of this 20 MHz span.
FIG. 2a is a polar plot of the amplitude response of antenna 5 to vertically polarized electromagnetic fields with respect to direction at the design frequency. In FIG. 2a, the further the response is from the center of the plot, the higher the amplitude gain of antenna 5 in that direction; hence, the easier antenna 5 will receive electromagnetic waves in that direction. Ideally, antenna 5 would have an equal response at every angle; this would indicate that the pager would receive signals from the base station equally well, no matter what direction the base station is at with respect to the pager. From FIG. 2a, it is apparent that the pager will receive signals easily at an azimuth angle of 270 degrees, but will have difficulty if the base station is located at 90 degrees azimuth.
Thus, what is needed is a pager antenna that overcomes the deficiencies and problems described above.